Conventionally, as a method of automatically correcting a tone of an image, there has been a method in which a brightness level of an image is corrected for each of pixels included in the image. An example of such method is described in JP-A-9-065252 (counterpart is: U.S. Pat. No. 5,808,697). In the method, an input image is divided into a plurality of block regions, and an average brightness level (an average of brightness levels of all pixels) is obtained for each of the block regions. Next, a correction curve is independently selected for each of the block regions corresponding to the average brightness level. Next, a plurality of types of correction curves selected for a block region (a main block region) including a target pixel and for a plurality of block regions (subsidiary block regions) which are adjacent to the main block region are subjected to a weighted average to generate a new correction curve, and the new correction curve is used to correct the brightness level of the target pixel. According to the method described above, it is possible to prevent the details of a local portion in the image from being lost by performing a tone correction on the image.
In the conventional method, the correction curve to be used for the correction of the brightness level of each pixel is obtained from the plurality of types of correction curves selected individually for the main block region and the subsidiary block regions. However, each of the correction curves is simply selected in correspondence with the average brightness level of each of the block regions (the main block region and the subsidiary block regions). Therefore, although the details of the local portion in the input image may be maintained by the tone correction, the effect obtained by the tone correction is rather limited.